176 DOC. 38 ETHER AND RELATIVITY 18 SIDELIGHTS ON RELATIVITY follows Mach, to the ether, which has to serve as medium for the effects of inertia. But this conception of the ether to which we are led by Mach’s way of thinking differs essentially from the ether as conceived by Newton, by Fresnel, and by Lorentz. Mach’s ether not only con- ditions the behaviour of inert masses, but is also conditioned in its state by them. Mach’s idea finds its full development in [19] the ether of the general theory of relativity. According to this theory the metrical qualities of the continuum of space-time differ in the environment of different points [20] of space-time, and are partly conditioned by the matter existing outside of the territory under consideration. This space- time variability of the reciprocal relations of the standards of space and time, or, perhaps, the recognition of the fact that “empty space” in its physical relation is neither homogeneous nor isotropic, compel- ling us to describe its state by ten functions (the gravitation potentials guv), has, I think, finally disposed of the view that